1. Field of the Invention
The invention relates to a method for removing paint from plastic parts coated with layers of paint.
2. The Prior Art
A method for removing paint from painted plastic parts is known from German Patent No. DE 43 44 582 A1. In this known process, the plastic parts are first mechanically crushed and subsequently circulated at an elevated temperature jointly with alkalinized glycol and in the presence of chemically stable abrasive particles. The alkalinized glycol causes the paint to swell, so that its adhesive power to adhere to the plastic parts is reduced accordingly. Due to the circulation of the plastic parts together with the abrasive particles, the swollen paint is scraped off from the plastic parts, which are subsequently rinsed with water, dried, and then separated from the scraped off paint particles. This known method achieves adequate removal of paint for most of the commonly used coats of paint.
However, the required circulation time can be kept within justifiable limits only by heating the paint removing agent to temperatures in the range of 80xc2x0 C. and 100xc2x0 C. with high expenditure of energy, which requires substantial amounts of paint remover in relation to the bulk material of crushed plastic pieces. This is because customarily employed paint removers, in particular alkalinized glycol, only lead to very poor wetting of the surface of the paint, which substantially reduces the duration for which the paint remover effectively acts on the coats of paint vis-à-vis the actual duration of the whole process. Furthermore, the abrasive particles required in this known process have a considerable size, so that they have to be completely separated from the now-naked plastic particles with substantial expenditure if the quality of the new plastic parts produced from such plastic particles is not be impaired.
In order to separate the naked plastic pieces from the paint removing agent, the plastic pieces are rinsed with water and subsequently dried. This requires substantial amounts of water. Furthermore, this treatment procedure renders the required drying process very costly in terms of energy.
It is therefore an object of the invention to provide a process of the type specified above in which the expenditure of energy required for removing the paint from painted plastic parts is reduced.
In connection with the method defined by the invention, the plastic parts consisting of thermoplastic material and having coats of paint are first crushed to bulk size, whereby at least 90 percent by weight of all pieces have an overall length of less than 20 cm following the crushing step. The plastic part from which the paint is to be removed is destroyed in this way. However, such crushing offers the advantage that the transport and treatment measures known in the field of bulk material technology can be applied without any problems to the plastic bulk material so produced. Furthermore, the predominating number of plastic parts from which the paint has to be removed have defects that make it impossible to directly reuse the entire plastic part.
However, the size of the pieces is selected to be larger than about 3 mm, preferably larger than 10 mm in order to keep the enlargement of the surface area of the plastic parts small by the crushing process. It is possible in this way to keep the amount of chemicals required for wetting the surface of the plastic pieces small as well. The goal of the method of the invention is to recover the plastic mass of the plastic pieces free of paint in order to be able to produce new plastic parts by known methods and to then paint such parts without the coats of paint adhering thereto being in the way. Since plastic components are often coated with thermosetting plastic paints, it is practically impossible to dissolve the coat of paint directly because the cross-linkage of the paint polymer would have to be broken up in this case. This, however, would require chemicals that may possibly attack the thermoplastic part itself, which should be avoided. A paint removing agent is therefore added to the bulk material of plastic pieces and circulated together with such bulk material. The paint removing agent causes swelling of the coat of paint and thus reduces its adherence to the plastic pieces. After the paint removing agent has acted for an adequate length of time, the adhesion of the coat of paint has been reduced to such an extent that it can be easily knocked off from the plastic pieces.
After the paint removing agent has acted on the coat of paint of the plastic piece, the paint removing agent is no longer needed for carrying out the subsequent steps of the method. In fact, the paint remover would interfere with the removal of the coats of paint from knocking, because it would dampen the knocks accordingly. Furthermore, there is always the risk that the paint removing agent might damage other processing equipment installed downstream. It is therefore preferable if the paint removing agent is separated from the plastic pieces after it has acted on them. It is basically conceivable to wash the plastic pieces with a simple solvent, in particular with water. However, this requires very large amounts of water and energy in the subsequent step for drying the plastic pieces. It is simpler and more efficient, on the other hand, if the separation between the paint removing agent and the painted plastic pieces is carried out by adding a solid substance absorbing liquid. Such a solid substance can be easily separated from the plastic pieces, for example by screening without requiring any substantial expenditure of energy for that purpose. As the paint removing agent is stored in the interior of the solid substance, it is possible also to employ chemically aggressive paint removing agents without causing damage to the treatment equipment downstream by residues of the paint removing agent.
In order to keep the energy expenditure as low as possible, the paint removing agent employed according to the invention is an emulsion comprised of at least one benzyl-substituted alkanol, an alkyl-glycol acetate and/or an N-alkyl-pyrrolidone with an aqueous lye. These components can not be completely mixed. The mixing ratio of the substances is selected so that it is in the range of a mixing ratio of between 1:2 and 2:1. In this way, the alkanol, the acetate or the pyrrolidone are prevented from completely dissolving in the aqueous lye, so that the result is an emulsion. Such an emulsion possesses high viscosity vis-a-vis the pure substances or the solution, so that a pulpy mass is obtained.
This provides the paint removing agent with high adhesive power for adhering to the painted surfaces of the plastic pieces, so that the pieces are wetted even if they are disposed in the circulation process far above any possible level of the liquid. This results in the advantage that the plastic pieces are wetted by the paint removing agent throughout the duration of the circulation process, which means that a short duration of action of the paint removing agent of about 7 to 8 hours is obtained even near room temperature. This eliminates the necessity of having to heat the paint removing agent or the plastic pieces in the course of the circulation process, so that the required feed of energy is reduced accordingly.
It is also possible to carry out the process for swelling the coats of paint at an elevated temperature, which reduces the required duration of action of the paint removing agent if the temperature is raised only slightly. A temperature rise to about 60xc2x0 C. is adequate for reducing the duration of action of the paint removing agent to a few minutes, which additionally results in substantial energy savings vis-à-vis the prior art. In addition, this results in the benefit that only a very small amount of paint removing agent of about 3 to 4% by weight of the fill of bulk material is required because the paint removing agent always precipitates in the form of a thin film. This means that it suffices to feed just as much paint removing agent that the plastic pieces are about uniformly wetted with the paint removing agent. Uniform wetting of the plastic pieces is achieved by circulating the plastic pieces, on the one hand, and by demixing the paint removing agent present in the form of an emulsion, on the other. In addition, the circulation could act on the plastic pieces in a scraping manner in order to detach the already swollen paint while the paint removing agent is acting on the plastic parts. This, however, is not necessarily required for carrying out the method in an effective manner.
The bumpers of motor vehicles are coated with highly impact-resistant, elastically yielding automotive paints in order to avoid damage to the lacquer from gravel impact. Such automotive lacquers are thermosetting paints, as a rule, which are applied to the plastic part on top of suitable primers for producing high power of adhesion. Such lacquers and primers are highly chemically stable as well. To remove the paint from such parts, it is advantageous if the emulsion contains benzyl alcohol, butyl-glycol acetate and/or N-methyl-pyrrolidone. These substances, moreover, offer the advantage that they have a relatively small molecule size, so that they can readily creep between the plastic parts and the coats of paint and thus effectively cause the coats of paint to swell.
Diluted soda or potash lyes have been successfully used for the lye component because these substances weaken the adhesive power of the paint adhering to the plastic parts in a particularly effective manner.
The paint removing agent as defined by the invention is basic, so that it can be disposed of in simple way via the sewer system. It is consequently necessary to neutralize the paint removing agent if such neutralization is carried out after the paint removing agent has acted on the plastic pieces by adding a neutralizing agent to the plastic pieces. This has the advantage that equipment installed downstream for treating the plastic pieces cannot be attacked by the basic milieu and possibly destroyed. This means that such processing devices can be made of favorably priced steel without reducing their useful life due to the action of the basic paint removing agent. An acid or a buffer solution has been successfully employed for neutralizing the paint removing agent. The acid acts substantially more effectively than the buffer solution if it is added to the amount of lye with the correct mixing ratio. No attention would have to be paid to the mixing ratio if a buffer solution is employed. However, the buffer is less efficient, so that a greater amount of neutralizing agent is required accordingly.
In order to obtain an adequate drying effect of the solid substance, it is important that such a substance has the largest possible active surface area. On the other hand, the required amount of solid substance should remain as small as possible in order to keep the costs of the process low. It has been found that solid substances having a surface area of at least 1 m2/g based on the mass are especially favorable for the method as defined by the invention. In combination with an addition of a small amount of solid substance to the plastic pieces, this results in an adequately high absorption effect of the solid substance for absorbing the paint removing agent.
It is basically very easy to separate the solid material from the plastic pieces by screening. However, individual particles of the solid substance may remain adhering to the plastic pieces. In order not to impair the repainting of a plastic component produced from the plastic pieces, the solid substance is preferably a powder with an average grain size of 20 xcexcm at the most, preferably 1 xcexcm at the most.
Precipitated silicic acid is preferably employed for absorbing the paint removing agent because it has a very large specific surface area of about 200 m2/g and, furthermore, a grain size of less than 1 xcexcm. Due to such a small grain size, residues of silicic acid on the plastic pieces will not interfere in any way with the later repainting of plastic components produced from such parts. Active carbon does in fact have an even larger specific surface area of about 500 to 800 m2/g. However, its grain size is relatively large, so that residues of active carbon on the plastic pieces may later interfere with the repainting under certain circumstances. Diatomaceous earth contains structures of minute living beings which cause it to have a substantial specific surface area of between 20 and 30 m2/g combined with a very small grain size. Soot, talcum powder or lime stone meal have a very small grain size of less than 1 xcexcm and their specific surface area is in the order of 1 m2/g, so that correspondingly larger amounts of the solid substance are needed for absorbing the paint removing agent.
If the paint removing agent and the solid substance are added in a closed container, the entire wet chemical process takes place exclusively in the container. This means that the entire remaining steps of the process can be carried out in the dry state, so that the processing devices employed downstream do not have to satisfy any requirements with respect to tightness and, furthermore, are not required to be equipped in any chemically resistant manner.
A uniform distribution of the solid substance through the bulk fill is obtained by circulating the plastic pieces together with the solid substance. This results in a uniform and almost complete removal of the paint remover, so that the subsequent treatment steps can be carried out in the dry state.
In order to keep residues of the paint removing agent that the solid substance has been unable to absorb away from the subsequent treatment equipment, the plastic pieces are subsequently dried. Such drying is preferably carried out via a wind sifting step in which the painted plastic pieces are subjected to a flow of a gas, particularly air blown through the plastic pieces from the bottom. Small particles, i.e. especially particles of the solid substance fully saturated with paint removing agent are carried along by the stream of gas, whereas the painted plastic pieces will remain in the wind sifting device. In this way, a separation takes place in addition to the drying process, which enhances the purity of the plastic recovered in the process accordingly. This is important particularly when the plastic components produced from the recovered plastic material are repainted.
Furthermore, it is possible in this way to recover at least part of the paint removing agent and possibly of the solid substance as well in order to reuse the materials so recovered.
The paint removing agent is capable only of swelling the paint or its primer without dissolving it because forming a solution is practically impossible in connection with thermosetting paints. To detach the paint from the plastic pieces, the paint is knocked off from the plastic pieces. A hammer or breaker plate mill that exerts knocks on the plastic pieces is preferably employed. Following an adequately long treatment of the plastic pieces in the hammer or breaker plate mill in the range of from 15 to 60 seconds, the plastic pieces are freed of the paint.
So that the naked plastic pieces can be reused without problems, it is important to separate the plastic from the paint particles and the solid substance absorbing liquid, so that a nearly pure plastic material is obtained. Such separation is preferably accomplished by screening because the plastic pieces are relatively large, whereas the knocked-off paint particles or the solid substance have a substantially smaller grain size. It is advantageous if a suitable sieve is already installed in the hammer or breaker plate mill in which the paint is knocked off from the plastic pieces. In a hammer mill so equipped, the paint particles removed from the plastic pieces, as well as the solid substance, but not the plastic pieces, are able to pass through the sieve. In this way, the plastic pieces are already separated from the paint or the solid substance in the hammer or breaker plate mill. So that the solid substance or the paint removing agent contained therein can be reused as well, it is advantageous if the solid substance is separated from the knocked-off paint, which is accomplished by screening as well. This is possible in an easy manner because the knocked-off paint particles have a substantially larger grain size than the solid substance. It is basically possible to install this sieve in the hammer or breaker plate mill as well, so that the latter separates the three fractions into xe2x80x9cplastic piecesxe2x80x9d, xe2x80x9cpaint particlesxe2x80x9d and xe2x80x9csolid substancexe2x80x9d. As an alternative, the paint particles can be separated from the solid substance in a screening device installed downstream. The paint particles collected in this manner have highly varying compositions because paints or lacquers consist of all kinds of different polymers, and small amounts of the plastic material are rubbed off in the treatment of the painted plastic pieces in the hammer or breaker mill as well, and are then received in or added to the paint fraction because of their small size.
Furthermore, the paints contain all sorts of different pigments, and thermosetting plastic to some extent as well, so that such plastics have to be disposed of. The naked plastic pieces may be reused either directly for producing new plastic components, or may be cut down first in a cutting mill to a grain size suitable for injection molding machines.
So as to be able to recover the paint removing agent and possibly also the solid substance, it is advantageous if the solid substance freed of the paint particles is heated to at least 70xc2x0 C., preferably at least to 100xc2x0 C. As the amount of solid substance is small in relation to the bulk fill of plastic pieces, the expenditure of energy required for this purpose is kept within justifiable limits. This elevated temperature partially evaporates the liquid absorbed in the solid substance from the latter, and this liquid can be subsequently condensed again for its recovery.
Especially if the paint particles are not to be separated from the solid substance, it is favorable if the solid substance loaded with the paint removing agent is heated to a temperature that is lower than the plasticizing temperature of the plastic pieces. The plastic pieces are in this way prevented from baking to the heating devices and thus from making it more difficult to carry out the method.
In order to keep the expenditure of energy for heating the solid substance low, it is favorable if the mechanical energy introduced for knocking off the paint from the plastic pieces is used for heating the solid substance. The solid substance is preferably heated by a heat pump that is supplied with the heat of the naked plastic pieces. It is possible in this way to advantageously use the substantial amount of heat contained in the naked plastic pieces for heating the solid substance, and to thus dispense with additional heat sources.
If the solid substance is additionally passed through a countercurrently operating heat exchanger with heating on the secondary side, the solid substance is heated in a particularly efficient manner with the lowest possible consumption of energy because the heat of the solid substance, which is at least partly freed of the liquid, is reused as well for heating the incoming solid substance.
Finally, it is favorable if the gas evaporating from the heated solid substance is removed by pumping it off. The atmospheric pressure is preferably reduced to about 100 to 500 mbar so as to assure that the evaporation from the solid substance is as effective as possible. The recovered vapor is preferably condensed in a cooler and subsequently separated in a phase separator into an oily and an aqueous phase, so that the organic component of the paint removing agent can be recovered for carrying out the method further. The aqueous phase contains water and a salt and can be discharged into the sewer system without problems.